Synthetic aperture radar, abbreviated SAR and sometimes referred to as a “look down” radar, is an instrument of choice for permitting aircraft and spacecraft, moving overhead, to monitor ground and naval operations occurring below. Such surveillance radar possesses the ability to “see” at night and the ability to see through fog and clouds, producing a map of the underlying surface topology as seen by the radar's emitted radar pulses. Presented on the radar's display, the map is of high resolution, permitting the radar operator to view essential buildings, ships, military tanks, trucks and other apparatus of military and commercial relevance on the surface underneath.
In common with other types of radars, synthetic aperture radar includes a radar transmitter, antenna, radar receiver and radar display, such as a cathode ray tube. Pulsed RF generated by the transmitter is emitted from the antenna and, following the transmission of each pulse, the receiver “listens” for echoes of such RF pulse, the radar echo, as may be reflected from a radar reflective surface. The receiver and ancillary circuits process the returned echo and output the processed echo to the display, where the images, composed from multiple echoes serially received, may be viewed by the operator.
As carried by an aircraft or spacecraft, the SAR's radar pulses are generated and directed by the antenna in a wide beam downwardly and to one or both sides of the craft, toward the surface below. Because of the motion of the SAR the wide beam sweeps a long, wide area on the ground. The SAR receiver and processor continuously interpret the returned echoes and categorize echo components according to Doppler shift and range so that, when many echo pulses are received and processed and presented on a radar display, an image of the ground and vehicles is produced. This process is continuous for the entire area swept by the antenna beam.
The SAR's radar display produces an image or map of the surface topology as a composite, revealing as part of such topology, any tanks, trucks and the like reposed in the region. If that mapping information is combined with navigational information, indicating the position of objects on the map, as may be obtained from satellite navigation systems as example, the exact geographical position of the object may be made available to weapons operators, whether aircraft or artillery, and the object is compromised.
The need to provide an effective camouflage for the ground based forces in such circumstances or to hide those forces from such unwanted surveillance is clear. However, apart from accessing and disabling the radar, until the present invention, there was no effective way to conceal one's activities from the radar's surveillance.
SAR's operate typically in the range of 3 to 35 GHz and reveal objects as small as a foot in length. The radar system measures two properties of the radar echoes from which the electronic display is constructed, range and Doppler shift. Each location on the radar reflective surface on which the RF pulse from the radar is incident, reflects an echo that has a unique set of range and Doppler components. That echo is then interpreted by the radar and is represented on the display in accordance with known technique. The present invention modifies the echo, which the SAR receives and interprets as the regular echo, and, thereby, produces a false picture on the radar display, one that obscures the details of the reflecting surface.
An object of the present invention, therefore, is to electronically camouflage targets, such as vehicles, troops, tanks, ships, and other radar signal reflective structures, to prevent detection and imaging by synthetic aperture surveillance radar; and
Another object of the invention is to create “snow” or smear on the synthetic aperture radar's display to conceal objects from the radar and prevent their identification.